T-Mobile US Inc. gained 1.4 million of the most lucrative wireless customers in the third quarter, but the cost of winning those gains hurt its margins.

The results extended a streak of strong growth for the carrier that kicked off the U.S. wireless industry's long-running price battle. The U.S. wireless carrier has added more than 5.6 million customers since the beginning of 2013 in a saturated industry with little real subscriber growth.

The report also keeps alive questions about how T-Mobile, the smallest of the four U.S. nationwide wireless carriers, can continue to post gains against better capitalized rivals.

T-Mobile's executives themselves spent much of the past year arguing the merits of getting bigger amid talks to merge with rival Sprint Corp. The results late Monday were the first since Sprint ended merger talks in early August and French wireless company Iliad SA dropped a takeover offer for T-Mobile earlier this month.

T-Mobile, which is majority-owned by Deutsche Telekom AG, reported a third-quarter net loss of $94 million, or 12 cents a share, compared with a loss of $36 million, or 5 cents a share, a year earlier. Revenue rose about 10% to $7.35 billion.

The company said Monday that it had higher selling, general and administrative costs in the quarter, along with higher losses on equipment sales.

Analysts polled by Thomson Reuters had projected a per-share profit of two cents and revenue of $7.4 billion.

The company's churn, or monthly service cancellations, fell to 1.6%, from 1.7% a year ago.

Write to Thomas Gryta at thomas.gryta@wsj.com

Subscribe to WSJ: http://online.wsj.com?mod=djnwires